1. Field of the Invention
The present invention relates to an outdoor water treatment apparatus to kill bacteria with ultraviolet light used primarily to treat water such as (bacterially) polluted water, swimming pool water, and fish farm (aquaculture) water that is outdoors and exposed to sunlight.
2. Description of the Related Art
Eradicating bacteria is essential in industry as well as in everyday life. In general, there are well known methods of killing bacteria (disinfection and sterilization treatments) such as chemical treatment, heat treatment, ultraviolet light treatment, and ozone treatment. However, with increasing awareness of the environment and chemical pollution, demand has developed for high quality treatment technology from perspectives that include not altering the treated substance, not leaving unwanted residues, and not detrimentally impacting the environment. In this context, treatment methods that kill bacteria with ultraviolet (UV) light are becoming widely adopted.
In contrast with chemical treatment methods, killing bacteria with UV light leaves no residues and is superior from the standpoint of safety. Further, since UV treatment damages bacterial deoxyribonucleic acid (DNA), it has the positive feature that chemical-resistant (antibiotic-resistant) bacteria are not produced as is possible in chemical treatments. The antibacterial mechanism associated with UV light exposure is commonly explained as follows. The cells of all living organisms as well as bacteria contain nucleic acid (including DNA) that administers cell genetic information. When irradiated with UV, nucleic acid absorbs the UV light and adjacent pyrimidine bases (primarily thymine) bond covalently to form a pyrimidine dimmer (a molecular lesion). This inhibits genetic transcription and metabolic disorder results leading to death of the cell.
An apparatus to disinfect water using ultraviolet light has been developed (refer to Japanese Laid-Open Patent Publication 2008-136940). The apparatus cited in this disclosure kills bacteria via a UV light source disposed outside a tube through which treatment water flows. The UV light source includes UV lamps (black lights) or light emitting diodes (LEDs) that shine UV light on the treatment water to kill bacteria. This apparatus can disinfect water without the use of chemicals. However, although UV disinfection damages cell DNA to render microbes inactive, subsequent irradiation with sunlight, which includes near ultraviolet (with wavelengths˜380 nm-200 nm) and visible (with wavelengths˜780 nm-380 nm) components, can cause inactive microbes to recover and multiply. Specifically, photoreactivation (also called photorecovery) can occur and reduce the effectiveness of UV disinfection. Photoreactivation is a result of activation, by light with a wavelength close to 400 nm, of genetic repair enzymes (photolyases) that repair pyrimidine dimmer lesions. Specifically, UV disinfection is due to chromosomal damage that results from direct microbial DNA impairment by pyrimidine dimmer lesion formation. However, when light with a wavelength close to 400 nm is shined on microbes rendered inactive by UV, genetic repair enzymes (photolyases) are activated that repair the pyrimidine dimmer lesions, and microbial reactivation occurs due to photoreactivation.
FIG. 1 is a graph showing microbial recovery due to photoreactivation after disinfection by ultraviolet light irradiation. In this graph, the horizontal axis is time in minutes, and the vertical axis shows the reduced number of Escherichia coli (E. coli) bacteria due to disinfection normalized to a value of one for the number of bacteria prior to disinfection. This graph shows the increase in the number of bacteria when the population is reduced to 1/2500th by approximately 30 min irradiation with UVC light having a primary emission peak at 254 nm and an intensity (power per area) of 0.01 mW/cm2, and subsequently is exposed to UVA light having a primary emission peak at 365 nm and an intensity of 0.30 mW/cm2. (Here, standard UV spectral range notation is adopted: UVA [315 nm-400 nm]; UVB [280 nm-315 nm]; and UVC [100 nm-280 nm].) From this graph it is clear that when E. coli bacteria are exposed to the UVA light included in sunlight after exposure to UVC light, the bacteria count, which was initially reduced to 1/2500th by disinfection, increases to 1/20th after approximately 180 minutes. The broken line in this graph shows the bacteria count reduced to 1/2500th by disinfection with no subsequent light exposure and no increase in the bacteria count. From this graph, it is apparent that disinfected E. coli bacteria increase by a factor greater than 100 due to photoreactivation.